This invention relates to machines for producing concrete pipe and other similar concrete products.
There are known and used in the industry numerous designs of machines for producing concrete pipe and other similar products. Some of these machines are single station machines, while others are multiple station machines. The latter type machines generally have three stations at which the basic cycles of filling, pressure-heading and stripping are simultaneously performed. At the first station, a jacket with a removable pallet secured to its lower end is lowered over a core, creating an annular space between the core and jacket which is then filled with concrete at the filling station. At the pressure heading station, a pressure head is lowered onto the top of the form to compact the concrete. At the third station, the jacket and pallet together with the now-formed concrete pipe is stripped form the core and moved to the curing area. The jacket is then released from the pallet and lifted from the now-formed pipe. A new pallet is then added to the jacket and the form is returned to the filling station and lowered over the core. The common technique for making concrete pipe is known as dry cast which produces a pipe of excellent quality at much higher production rates than the wet cast process. In dry cast, a dry mix is compacted and the pipe is removed promptly after the concrete is set but before the concrete is completely cured. An example of dry cast techniques used in making concrete pipe is shown in Schmidgall et al U.S. Pat. No. 4,356,628.
Concrete pipe are sometimes manufactured with a plastic liner that provides increased resistance to corrosion and deterioration from various chemicals in and gases emitted from liquids flowing through the pipe. The plastic material used for lining concrete pipe is extruded in a sheet form and is typically provided with T-shaped ribs that project outwardly from one side. These T-shaped ribs become embedded in the concrete during the pipe making process, and when the concrete is set, an excellent bond is created between the liner and the finished pipe. However, it is not uncommon for the T-shaped ribs of the liner to pull out away from the concrete during the casting process. This occurs in the dry cast process because the concrete is set but not completely cured when the product is stripped from the core. Not infrequently, this results in a bulge or pullout because of the friction that is created between the liner and the core when the core is removed. Moreover, when the dry cast process takes place using a rigid non-collapsing shape of core, it is also difficult to place the liner over the core because the liners are large and flexible and pre-formed into a tube that must fit tightly over the core. In an attempt to overcome the problems of pullout and bulging in the plastic liner that may occur when a rigid non-collapsible core is used, collapsible and expandable inner cores have been developed and are typically used in the dry cast method. When collapsible cores are used, the core is collapsed to allow the liner to more easily be placed over the core after which the core is expanded and the pipe is cast. The core is then collapsed to permit easy removal of the finished concrete pipe. An example of a pipe making machine for making lined pipe using a collapsible core of this type is shown in Schmidgall U.S. Pat. No. 5,720,993.
At the present time, the core is placed on a pallet at a setup area, and the plastic liner is manually placed over the core. The jacket is then lowered over the core with the liner in place. Then, this core-pallet-jacket module is transported to the pipe making machine to be filled with concrete. After being filled with concrete, the module is moved to the pressure heading station, where the pressure header is lowered to compact the concrete. As this step in the process is performed, the header will bear against the core to center it with respect to the jacket. However, during this pressure heading step, it is possible for the header to snag the plastic liner, and as the header is moved into position, the header may also scrape against the liner and damage it. If the damage is not repaired, the pipe will be defective, because when the pipe sections are assembled in the field, the interior concrete surface of the pipe at the point of the damage will be exposed to the chemicals in the liquid flowing through the pipe. Therefore, the damage must be repaired manually by hot air welding a plastic patch over the damaged area. Obviously, this type of repair is a time consuming and difficult process because a worker has to work inside the pipe to make the repair. In producing pipe with plastic liners, some pipe manufacturers use the same standard-size headers that are designed for producing pipe without plastic liners. In this case, the liner is not contacted by the header and must be cut short since the inside diameter of the header is too small to pass over the liner. When lined pipe produced in this manner are installed in the field, there is a gap in the liner where two sections of pipe are joined. This gap must be covered by a wide annular band of plastic that is hot-air welded around both edges of the liners of the adjoined pipe sections. This is a difficult job because a worker now has to crawl inside the pipe to the area where two sections are joined and apply the band. To make this job easier, many manufacturers will use headers having an inside diameter large enough to slip over the liner, the end of which will now extend through the header and beyond leaving a flap of the liner long enough to extend over the liner of an adjoining pipe section when they are assembled in the field. Although the liner must still be hot-air welded along one edge, the flap eliminates the necessity of an annular band requiring two edges to be welded to adjoining pipe sections. However, because the header must now pass over the end of the liner during the pressure heading step, the header must be guided over the liner to prevent snagging with resulting damage to the liner. At the present time, the header is guided over the liner by two or more production workers each using a tool, such as a trowel, to guide the header. Obviously, this requires additional labor and slows down the pipe making process. Therefore, there is a need for an improved way of protecting the liner from damage during the pressure heading step in the pipe making process.
It is therefore the principal object of the invention to provide a method and structure for protecting the plastic pipe liner during the pressure heading step of making the pipe, and thereby produce a finished product of higher quality while also increasing the productivity of the pipe making process and reducing the cost of producing the pipe.